1. Field of the Invention
The present invention relates to a driving circuit for an organic light emitting display apparatus, a display panel having the driving circuit and a display apparatus having the driving circuit.
2. Description of the Related Art
Flat display apparatuses such as organic light emitting display (OLED) apparatus have low cost, thin thickness, light weight and so on have been developed.
The OLED apparatus does not require a backlight assembly so that the OLED apparatus has thinner thickness and lighter weight compared with a liquid crystal display (LCD) apparatus. The OLED apparatus may be manufactured by lower cost compared with the LCD apparatus. Furthermore, the OLED apparatus has wider viewing angle and higher luminance compared with the LCD apparatus. The OLED apparatus displays an image using light generated by an organic electroluminescent element. When an electric energy is applied to the organic electroluminescent element, light is generated from the organic electroluminescent element.
The OLED apparatus is classified into an active-matrix type OLED apparatus and a passive-matrix type OLED apparatus. The active-matrix type OLED apparatus includes a switching element corresponding to a unit pixel of the OLED panel.
The unit pixel of a conventional active-matrix type OLED apparatus includes a switching transistor (QS), a driving transistor (QD), a storage capacitor (CST) and the organic electroluminescent element (EL).
In general, brightness of the OLED apparatus is lower than that of a cathode ray tube (CRT) apparatus. The active-matrix type OLED apparatus, however, has brightness higher than that of the passive-matrix type OLED apparatus. An amount of the light generated from the organic electroluminescent element increases in proportion to a current density of the current applied to the organic electroluminescent element.
A hydrogenated amorphous silicon transistor (a-Si:H) has a mobility lower than that of a poly silicon (Poly-Si) transistor. In addition, the amorphous silicon transistor generally does not employ a p-type transistor because it is difficult to form the amorphous silicon transistor using the p-type transistor. Furthermore, the amorphous silicon transistor has a unstable bias stability. Therefore, the OLED apparatus generally employs the poly silicon (Poly-Si) transistor rather than the amorphous silicon transistor. The poly silicon transistor, however, is more expensive than the amorphous silicon transistor.
When a driving circuit for driving the electroluminescent (EL) element has the amorphous silicon transistor, the driving circuit employs only an n-type transistor. When an OLED apparatus such as the active-matrix type OLED apparatus displays the image using the current, the current flowing through the electroluminescent (EL) element is controlled so as to express gray scales of the image.
A channel conductance corresponding to a gate-source voltage (Vgs) of the driving transistor (QD) is controlled in response to a data signal that is applied to a gate electrode of the driving transistor (QD) so as to control the current flowing through the electroluminescent (EL) element in response to the data signal that is provided from external image source to the OLED apparatus. The electroluminescent (EL) element is electrically connected to a thin film transistor (TFT) (or a driving transistor (QD)) in series.
When the OLED apparatus includes the p-type transistor as the driving transistor (QD), the electrode, of the driving transistor (QD), connected to a bias voltage line functions as a source electrode since the bias voltage line has a high voltage, and a size of the gate-source voltage (Vgs) between the gate electrode of the driving transistor (QD) and the source electrode of the driving transistor (QD) is determined by a voltage that is applied to the gate electrode of the driving transistor (QD) via a data line (DLn).
When the OLED apparatus includes the n-type transistor as the driving transistor (QD), the electrode, of the driving transistor (QD), connected to the electroluminescent (EL) element functions as the source electrode so that a voltage applied to a node (N1) that is electrically connected to the driving transistor (QD) and the electroluminescent (EL) element may be unstable. The voltage applied to the node (N1) may be changed in response to the data voltage signal corresponding to a previous frame. In addition, a dynamic range of the gate-source voltage between the gate electrode of the driving transistor (QD) and the source electrode of the driving transistor (QD) is narrower than the dynamic range of the data voltage signal that is provided from external image source.
Therefore, the driving transistor (QD) of the OLED panel generally employs the p-type transistor instead of the n-type transistor.